This invention relates in general to switching devices and in particular to a switching device of a type which includes a plunger which is guided for reciprocating movement in a housing, the plunger having at least one recess or opening defining therewith two opposite abutment surfaces and accommodating a first compression spring, and a contact arm arranged in the opening and extending transversely to the direction of movement of the plunger between at least a pair of stationary contacts mounted on the housing.
Switching devices of this kind find application in many switching systems of different construction and in many fields of application. As an example of such known systems, there may be mentioned limit switches which in practice are manufactured in large series. A common feature of these switching devices is a reciprocating plunger which is actuated either manually or mostly mechanically and which serves simultaneously as a support for the contact arm or contact bridge piece. In such switching devices, there can be provided only one contact bridge or several contact bridges arranged side-by-side or one above the other. For the sake of switching reliability, the stroke of the plunger or of the support of the contact bridge must be larger than the path of movement of the contact pieces on the contact bridge. In addition, the contact pressure between the movable and stationary contact pieces cannot correspond to the actuation force applied to the plunger. For these reasons conventional switching devices include a pressure spring which upon actuation of the plunger returns the same into its rest position, where it remains until its reactuation. Another pressure spring is arranged in the recess in the plunger in engagement with the contact bridge. These two springs determine together the contact pressure which is accurately adjustable to a desired value. Each contact bridge, therefore, is loosely supported in the recess of the plunger and is held in position by the resilient force of the pressure spring. This prior-art arrangement, however, has the disadvantage that, during the switching action, an electric arc may occur leading to local overheating at contact points between the movable and stationary contact pieces. This overheating, on the other hand, causes an undesirable adherence or even welding of the contact pieces to each other. It may therefore happen during the actuation of the plunger that one or the other movable contact sticks to the adjoining stationary contact piece and does not open either at all or else only upon the displacement of the contact bridge over a major part of the working stroke when the contact bridge is brought into an oblique position and that the pressure spring is eventually capable of tearing the two adhered or welded contact pieces apart. It is evident that, for these reasons, prior-art switching devices of this kind do not ensure a sufficient operational safety and reliability.